Embolic particles carried by the bloodstream can causes strokes and other circulatory disorders. During surgery emboli may occur when clots form in the blood, air enters into the bloodstream, or tissue fragments break loose or become dislodged. The blood carries the emboli into increasingly smaller arteries until they become lodged and obstruct the flow of blood. The amount of damage that results depends on the size of the emboli, the point in which it lodges in the blood flow, the amount of blood leaking around the emboli, and how blood is supplied by collateral paths around the obstruction. The resulting functional deficit depends in part on the composition of the emboli. For example, air may be reabsorbed in a short time, clots may dissolve, (particularly if blood-thinning drugs are present), while particles composed of plaque and body tissue may not dissolve at all. Therefore, it is important to have non-invasive instrumentation that can accurately detect the presence of emboli, determine their composition, and estimate their size so that appropriate medical management decisions can be made.
Instrumentation for detecting and classifying emboli based on broadband ultrasound is described in U.S. Pat. No. 5,441,051, the disclosure of which is incorporated here by reference. When an emboli passes through an ultrasound beam, the change in acoustic reflectivity causes a reflection which can be detected by an ultrasound receiver. In the '051 patent, the number of embolic events can be counted by monitoring the number of reflected echoes that exceed a predetermined threshold. The '051 patent also describes a method to characterize emboli by composition and size so that an embolus may be classified for example as a gas or a fat particle based on detailed analysis of the echo signal for each embolus.
Before moving objects can be accurately counted and classified, reflected signals from the moving object need to be processed to eliminate reflections from stationary objects that are of less interest. In the blood scanning application, these stationary objects include the blood vessel walls and surrounding tissue. The reflections from surrounding tissue are generally stronger than those from the flowing blood and from the emboli. The strong reflections from stationary objects may be reduced using a moving object indicator (MOI). An MOI temporarily stores one line of echo data and subtracts it from a subsequent line of echo data. Differencing two lines of echo data substantially cancels the stationary object signals leaving the signal reflected from the moving objects, e.g., from the blood flow and the emboli contained therein.
The noise performance of an ultrasonic moving object indicator is a significant issue. One way of improving noise performance is to average multiple lines in such a way that the signal-to-noise ratio is improved. In that case, differences are determined between the averages. The signal-to-noise ratio improves by a factor of the square root of the number of lines averaged when the noise is incoherent and the reflected signal is coherent. Averaging multiple lines results in a waveform that responds slowly to changes. The averaged waveform does not change significantly even when a moving object, e.g., an embolus, passes through the ultrasound beam. The differencing, however, produces a large value when the moving object is present in the ultrasound beam. In addition, the averaging “filter” still leaves significant background noise artifacts.
The inventors realized that the performance of an ultrasonic moving object indicator could be significantly improved. They also recognized that traditional signal-to-noise ratio enhancements are very computationally intensive and thus may often require a specialized or expensive hardware and/or software. In addition, detection of embolic signals based on exceeding a predetermined threshold is not coherent from pulse to pulse, as the amplitude of the return signal often varies with time. Incoherent detection reduces the accuracy of subsequent line tracking and object classification. The echo signal enhancements described below overcome all of these problems.